This invention relates to control apparatus for a three phase alternating current (AC) load which, in combination with manually or magnetically operable means prevents overloading and load burnout; protects the load power supply; and otherwise functions as a monitoring arrangement for the circuitry involved.
Prior to the present invention load protection means for the purposes described included, in combination, bimetallic relays/switches and/or circuit breakers/fuses. In this connection it is noted that for a load such as a three phase motor the motor starting current is five to ten times that of the motor running current. Accordingly, the load protection means used required a capacity to accomodate the higher starting current over a predetermined range, depending on the motor starting time.
With a three phase motor, when one phase malfunctions the motor starting or running current will increase by at least the .sqroot.3 times the normal starting or running current, as is well known by those skilled in the art. This condition leads to motor overload. While prior art protection means interrupts the power to the motor, a time delay for this interruption is experienced due to the heretofore mentioned motor starting current range. This can result in motor burnout, or in a malfunction of the motor power supply.
In regard to a motor power supply malfunction, consider a 100 ampere power supply driving a three phase motor. Usually, each of the motor phases is balanced relative to the other phases but not to the power supply. For purposes of example, one phase of the power supply may draw 75 amperes while the other two phases may draw 85 amperes. If one phase of the power supply is lost the remaining two phases will drive the motor causing overloading of said two phases by at least .sqroot.3 times the normal current. It will be recognized that the current will be in excess of 100 amperes causing a malfunction of the power supply.
Further, consider for example a three phase resistance load such as a heater. If one phase is lost the heater output will decrease. The present invention is arranged to cut off power to the heater so that the heater or the input power supply can be checked to determine the cause of the reduced heater output, and in this sense the present invention acts as a monitor.
The applicant is aware of U.S. Pat. Nos. 4,647,825; 4,550,360; and 4,297,741 which were cited during the prosecution of the aforenoted U.S. application Ser. No. 046,455.
U.S. Pat. No. 4,647,825 which issued to Profio, et al on Mar. 3, 1987 relates to a motor controller with means responsive to a particular motor condition for preventing termination of power to the motor in response to fault conditions when these fault conditions occur during motor start up. Hence, a phase load detector receives an input signal from a converter. If there is a loss of one phase after the motor is up to speed the phase load detector generates a signal to open a motor relay. Thus, the Profio device is seen to relate to a distinctly different idea of means than that of the present invention.
U.S. Pat. No. 4,550,360 issued to Dougherty on Oct. 29, 1985 relates to a circuit breaker static trip unit and an analog signal processor within said unit which is provided with an A/D converter, a digital processor and an electrically alterable non-volatile read only memory (EAROM) for trimming and calibrating the trip unit. Likewise, this patent relates to a distinctly different idea of means than that of the present invention.
U.S. Pat. No. 4,297,741 issued to Howell on Oct. 27, 1981 relates to air core current transformers linked with the phase conductors of a power distribuiton circuit and which develop secondary voltage signals proportional to the rates of change of the phase currents. A comparator issues a circuit breaker trip initiating signal when the output voltage signal achieves a preselected threshold value. The disclosed invention is not seen to relate to the present invention.